As a homebrewer you'll typically want to know how much alcohol is contained in the latest batch of precious beer that you have fermented, and thankfully the process for working this out is fairly simple.
As we know, the process of brewing beer involves extracting the sugars from malted grains (and sometimes other fermentable ingredients called adjuncts). It is these sugars that are consumed by the yeast during fermentation that create (amongst other things) the byproducts ethanol (alcohol) and carbon dioxide gas.
In order to calculate how much alcohol is in the finished beer, we need to know how much sugar has been consumed by the yeast - so we take gravity readings before and after fermentation to determine how much sugar is in the wort (unfermented beer) and the finished product, and run a simple calculation on these numbers to determine alcohol content.
How is gravity measured? There are several tools or instruments that can be used to do this, but by far the simplest and cheapest is to use a floating hydrometer (pictured below). Simply take a fluid sample in the sample tube, and insert the floating hydrometer into the tube. It will float and show the gravity reading where the stem protrudes from the sample solution.
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| An example of a floating hydrometer reading 1.000 in plain water |
Gravity is measured as a decimal number, with 1.000 being the gravity reading of water. As sugar is added to water, this number will increase. The higher the number, the more sugar is present in the water. A typical starting gravity is referred to as Original Gravity (OG) and in brewing terms is often around 1.040-1.060 for most beer styles. There are of course exceptions on either side of this, but this range is a rough starting point to give some perspective on the numbers we're talking about.
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| A floating hydrometer reading an Original Gravity (OG) of around 1.064 |
Original gravity can be measured using a floating hydrometer or refractometer with fairly accurate and reliable results (assuming you're hydrometer is accurate/calibrated correctly to begin with). This can be fairly easily checked by measuring water to confirm your gravity reading device reads 1.000.
At the end of fermentation, a subsequent gravity reading is taken, with the number expected to be much closer to 1.000. This is referred to as a Final Gravity (FG) reading. The lower this number is, the more sugar has been consumed by the yeast and the more alcohol will be present in the finished beer.
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| An example of a digital refractometer |
Since the finished beer has alcohol present in it, refractometers are not as accurate/reliable for obtaining readings as the alcohol will distort the refraction of light, leading to inaccurate results. There are calculators available to account for this, but we'd recommend sticking with the aforementioned floating hydrometer as it's still an accurate method for measuring the final gravity of beer when alcohol is present (without any additional calculations), so this is your best bet especially when starting out.
The formula you need to use is the difference between the Original Gravity (OG) and the Final Gravity (FG) multiplied by 131.25, which will give you the Alcohol By Volume (ABV) as a percentage.
For example, if our OG was 1.045 and the FG was 1.010, the formula would be;
(OG - FG) x 131.25
(1.045 - 1.010) x 131.25
.035 x 131.25
= 4.59%
Pretty simple in the end, and if you don't feel like breaking out your calculator there are loads of online calculators you can use where you just enter the OG and FG values and click a button and it will give you the reading.
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